[RFC PATCH] ptrace: add PTRACE_GET_SYSCALL_INFO request

[Andy Lutomirski]

> On Nov 7, 2018, at 8:44 AM, Oleg Nesterov <oleg at redhat.com> wrote:
> 
>> On 11/07, Andy Lutomirski wrote:
>> 
>> 
>>>> On Nov 7, 2018, at 3:21 AM, Oleg Nesterov <oleg at redhat.com> wrote:
>>>> 
>>>> On 11/07, Elvira Khabirova wrote:
>>>> 
>>>> In short, if a 64-bit task performs a syscall through int 0x80, its tracer
>>>> has no reliable means to find out that the syscall was, in fact,
>>>> a compat syscall, and misidentifies it.
>>>> * Syscall-enter-stop and syscall-exit-stop look the same for the tracer.
>>> 
>>> Yes, this was discussed many times...
>>> 
>>> So perhaps it makes sense to encode compat/is_enter in ->ptrace_message,
>>> debugger can use PTRACE_GETEVENTMSG to get this info.
>> 
>> As I said before, I strongly object to the use of “compat” here.
> 
> Not sure I understand you... I do not like "compat" too, but this patch uses
> is_compat/etc and I agree with any naming.

My point is: returning a value to user code that is:

0 if the kernel and tracee are 32-bit
0 if the kernel and tracer are 64-but
1 if the kernel is 64-bit and the tracer is 32-bit
? If the tracer is arm64 ILP32

Is not a good design.  And 32-bit builds of strace will not appreciate it.

The API should return a value that, at least on a given overall architecture and preferably globally, indicates the syscall arch.  While oddly named, audit_arch fits the bill nicely, and we already require it to have the right semantics for seccomp support.